GNSS receiver systems can increase their sensitivity by integrating the received GNSS signals over a longer period of time, so long as the time base used within the receiver is sufficiently stable over that time period. The stability of the crystal oscillator used in low-cost commercially-available consumer-grade GNSS receiver devices restricts the coherent integration time to below 10 seconds. Consumer GNSS devices are often used in mobile environments, resulting in timing changes due to their motion.
A typical GNSS receiver system may not have up-to-date satellite information and thus would need to perform an extensive search of frequency and code offsets for each satellite in order to synchronize its time to the GNSS time and achieve lock. FIG. 1 shows the space that is searched by a conventional GNSS system, such as a GPS.
When a GNSS receiver initially locks to a satellite, it needs to scan frequency offsets and code offsets to account for the drift in the receiver's time base, which is often provided by a low-cost crystal oscillator. This search must be repeated if the GNSS receiver stops tracking signals from the GNSS satellite system which may happen for a number of reasons. The coherent integration time used by consumer-grade GNSS receivers is limited by a variety of factors including stability of the timebase at the receiver, and the prohibitive cost, power and size of a timebase which would enable longer integration times. If a GNSS receiver loses lock for some period of time (such as when it is indoors), its time base will quickly drift away from the GNSS time reference. Such drifts prevent the receiver from acquiring the GNSS signals or performing coherent integration over longer periods of time during acquisition.